Hi everyone,
Please check out my data correlating light and screen flicker with my concussion-like health symptoms posted here: https://www.flickersense.org/testing-leds-and-screens
I bought a professional UPRTek meter and have tested hundreds of light bulbs (dozens of kinds at least). I used this meter on screens too, but ended up also using microscopes and slow-motion video to quantify screen flicker better in 2 dimensions.
My basic conclusions are that for me, flicker is bad, color-to-color-flicker is much worse, and with limited data, there's a hint that randomness in flicker might reduce harm from flicker.
It turns out that the "good" Phillips LEDs I've used without harm since 2013 have 1% flicker, no color-to-color flicker, and happen to be in a fixture with a slightly unstable electrical supply that adds a little randomness to the flicker. So it's possible it's that randomness, rather than the bulbs themselves that makes them OK for me. Except for these Philips bulbs, I'm back to using incandescents at home.
All the screens I tested have significant flicker, especially at the subpixel level. It turns out that all of my screens flicker the red, green, and blue subpixels in order to decrease color saturation - so the amount of flicker varies for different colors. This happens at 15 Hz, 30 Hz, or 60 Hz depending on the screen. The 15 Hz LCD screen seems much more harmful to me than 30 Hz LCD screens. It's worse for me when there's more subpixel flicker and especially bad when the red, blue, and green flicker don't track with each other, creating color-to-color flicker.
Every screen I tested does something different with flicker, so it doesn't surprise me at all that sometimes those of us on this forum experience different things with different screens and it's hard to figure out general trends. Although one trend is that on all of my Apple devices, Nightshift works by adding somewhat more green and a lot more blue subpixel flicker, thus reducing the green-blue light. Now I know why my brain gets especially injured by Nightshift!...Why does Apple do this??